1. Field of the Invention
This invention relates generally to the exposure to light of a predetermined pattern on a surface of a photo-sensitive member, and more particularly is directed to an improved method for effecting such exposure in a pattern composed of a large number of parallel, fine stripe-like areas.
2. Description of the Prior Art
There are various instances in which photo-etching techniques are employed for producing a fine striped pattern of a suitable material on a substrate. For example, in producing the target of a so-called storage tube, a layer of silicon is deposited on the inner surface of the face plate of the tube envelope to form an electrode, and the silicon layer has its surface oxidized to form an insulating layer of silicon dioxide thereon. Then, the silicon dioxide insulating layer is photo-etched to selectively remove portions thereof and leave a pattern of parallel, spaced apart insulating stripes between which the silicon electrode is exposed for impingement thereon by an electron beam when the latter scans the target in the direction transverse to the insulating stripes. However, since the pattern of insulating stripes is very fine, for example, each stripe may have a width of 5 microns and the pitch between adjacent stripes may be 10 microns, great difficulty is experienced in accurately producing the photo-mask to be used in connection with the photo-etching of the silicon dioxide insulating layer. Similar difficulties are encountered in producing a photo-mask for use in the attainment, by photo-etching techniques, of a pair of comb-shaped electrodes having patterns of interfitting, parallel, spaced apart stripe-like electrode elements on the target structure of an image pickup tube, for example, of the type disclosed in U.S. Pat. No. 3,772,552, issued Nov. 13, 1973, and having a common assignee herewith.
In producing a photo-mask for use in forming the above mentioned fine patterns of insulating stripes or stripe-like electrode elements by photo-etching, it has been proposed to first make a master drawing on a stable paper of only a small segment of the desired pattern which is drawn on a greatly enlarged scale, for example, 1000 to 2000 times actual size. Then, an original photo-mask is optically produced from the master drawing with suitable reduction of the scale thereof so that the widths of the several light-permeable stripes which are of uniform lengths and the pitches between the stripes on the original photo-mask are maintained within desired tolerances. The resulting original photo-mask is employed in a so-called photo-repeater in which light is directed from a suitable source through a shutter and the pattern of light-permeable stripes on the master photo-mask against a photographic dry-plate or other photo-sensitive medium supported on a stage which is movable relative to the original photo-mask in orthogonally related directions extending parallel and transversely, respectively, to the direction of the light-permeable stripes. The stage is repeatedly scanned or moved relative to the original photo-mask in the direction parallel to the light-permeable stripes with the shutter being open only during each of such scanning movements and, in the intervals between the successive movements, the stage is displaced or shifted relative to the original photo-mask in the direction transverse to the light-permeable stripes by a distance corresponding to the effective width of the pattern of light-permeable stripes on the original photo-mask in such direction of shifting. Thus, at the completion of the repeated scanning movements, light passing through the light-permeable stripes of the original photo-mask will have exposed a relatively large number of stripe-like areas on the photo-sensitive medium with the lengths of such exposed stripe-like areas being substantially greater than the uniform lengths of the light-permeable stripes of the original photo-mask. When the exposed photo-sensitive medium is developed, the resulting photo-mask for use in a photo-etching process has the desired relatively large number of parallel transparent stripes which are separated by opaque or light shielding portions.
However, in the photo-mask obtained by the existing method, as described above, unacceptably large inaccuracies may occur in the pitch between adjacent transparent stripes which correspond to the last and first stripe-like areas, respectively, exposed on the photo-sensitive medium in two successive scanning movements of the stage relative to the original photo-mask. Such unacceptably large inaccuracies result from the inability to more precisely position the stage when the latter is shifted between successive scanning movements. If the target of a storage tube is produced by photo-etching with a photo-mask having relatively large variations in the pitch between certain adjacent transparent stripes of the photo-mask, the resulting insulating stripes on the target have corresponding pitch variations which, when a picture or image is reproduced on the storage tube, will appear as stripes causing deterioration of the picture quality.